MR imaging systems are used in particular for the examination and treatment of patients. By such a system, the nuclear spins of the body tissue to be examined are aligned by a steady main magnetic field (B0 field) and are excited by RF pulses (B1 field). The resulting relaxation signals are exposed to gradient magnetic fields for the purpose of localization (or slice selection) and are received and evaluated in order to form in a known manner therefrom a one-, two- or three-dimensional image of the body tissue.
Essentially two types of MR imaging systems can be distinguished. The first one is the so called open MR imaging system (vertical system) in which a patient is introduced into an examination zone which is situated between the ends of a C-arm. The patient is accessible during the examination or treatment from practically all sides. The second one is a MR imaging system which comprises a tubular (cylindrical) examination space (axial system) into which the patient is introduced.
RF coil systems are provided for the transmission of the RF pulses (B1 field) and the reception of the relaxation signals. In addition to the RF coil systems which are permanently built into the MR imaging apparatus (body coils for imaging substantially the whole body of a patient), use is also made of RF surface coils which can be flexibly arranged, for example, as a sleeve or pad around or in a specific region to be examined. Similarly, RF head coils are provided and adapted for the examination of a head of a patient.
Furthermore, use is made of catheters and implantable cardiovascular devices or other invasive or interventional devices which are introduced into the patient, for example, in order to take a sample of tissue or for RF ablation of tissue during the MR imaging and which comprise at least one coil element, an oscillator or the like at least at the area of their tips for the purpose of localization in the reproduced image or for the purpose of imaging.
For connecting the above distal electrical units with a connection or base unit, notably a power supply or a control or evaluation unit (which can be a part of the related MR imaging system or a separate part) electrically conductive links or connection leads or lines or cables usually have to be guided through the above mentioned (strong) RF electrical and/or magnetic fields of the MR imaging system or MR scanner.
Especially RF fields can induce RF common mode signals (currents) in the connection line and in the surrounding body tissue of a patient. These currents involve not only the risk of disturbances or destruction of the distal and/or of the connection unit, but notably can give rise to substantial heating of the line and of the adjacent body tissue and, especially in the case of surface coils and catheters, to burning of the patient when the connection line is too close to or within the patient. Consequently, such lines have to be made RF safe with respect to these risks.
U.S. Pat. No. 6,496,714 discloses an RF safe invasive device with at least one long conductor for use in a magnetic resonance imaging apparatus. The invasive device is provided with at least one series element of controllable impedance especially in the form of a diode, incorporated into the long conductor where the impedance of the series element is controlled by a control unit such that during RF transmission the series elements subdivide the long conductor into sections substantially shorter than half the wavelength in tissue of the RF used. By this, standing RF waves along the conductor during RF transmission, and the associated dangerous heating of the device and surrounding tissue shall be avoided.